Collections List Page
To customize the 'Collections List' page (your-storename.com/collections), open Theme Editor (Shopify Admin > Themes > Customise), select 'Collections List' from the dropdown located in the top area.
Click on 'Collections List' section to adjust settings. By default, all collections (except the one collection named 'Frontpage') will be displayed on this page. You are also able to control which collections you want to be present on this page, to do so change 'Select collections to show' setting from 'All' to 'Selected' and click 'Add collection' button below to select a collection you want to be displayed. Also, you can specify how many collections should be displayed in a row with 'Collections per row' setting.